What's selection bias?
- Selection on IV
- Selection on DV
Ideal situation
Different types of selections
Selection on Both
The consequence of selection bias
- When your case study suffers from selection bias,
- Any characteristics that the selected cases share is a cause.
- The relations between variables within the selected set reflect a general pattern.
Example
- Question: Why new industrializing countries (NICs) grows more rapidly?
- Theory: The repression of labor
- Case:
- What type of design this is?
What's the problem?
When selection is not a problem
- Theory-oriented
- Labor repression contributes to growth in a at least mixed economy
- Case selection: Communist countries are excluded.
- Hypothesis-oriented
- Theory: Military governments are more likely to negotiate their extrication from power than are personalist regimes.
- Hypothesis: Incidence of negotiation in military governments during the years in which breakdown occurs is higher than that in personalist regimes.
- Case selection: Only breakdown regimes are select.
Avoid selection bias (when it's possible)
- Being clear about the domain of the variables
- Coding sheet
Example from Skocpol
Ask a correct question
- "What was the effect of cause X?" \(\checkmark\)
- "What cause Y?" or "Variance of the effect of X?" \(\times\)
Toolbox of case study
Least-likely case
- If the theory can make it here, it can make it anywhere.
- E.g.: Evangelista(1999)
- Target: Influence of transnational actor
- Theory: Transnational actor can affect international relations
- Case: U.S. vs. USSR
- Scientists' contact affect the course of U.S. and Soviet defense and arm control policies.
Most similiar systems
- Control similiarity, focus on differences.
- E.g.: Ray(1995)
- Target: Interstate conflicts
- Theory: Democratic peace
- Case: UK-France Fashoda Crisis vs. Spanish-American War
- Control for confounders (regression effects, mortality, selection bias) and same year
- Explanatory: regime
Deviant Cases
- Cases that do not conform to the predictions made by the theory or theories under investigation.
- E.g.: Elman(1997)
- Theory: Democratic peace
- Case: Finland vs. UK in the Continuation War
- Finland: Legislature does not balance president
- Conclusion: decentralized vs. centralized democracies
Process tracing
- Explicit attention to and process tracing on alternative explanations
- Sustained focus on the question of "what else must be true" of the process
- A wide variety of sources